In many applications, downhole assemblies which are quite lengthy need to be inserted into live wells. One technique that has been used in the past to accomplish this is to assemble a very tall lubricator. A lubricator is an isolation device mounted at the surface, which allows, through sequential valve operation, the assurance of a chamber which is at least doubly isolated from wellbore pressure, so that lengthy downhole assemblies can be assembled therein. Once the lengthy assemblies are fully put into the lubricator, the lubricator is isolated at the top around tubing or wireline and opened at the bottom. The tubing or wireline is then used to advance the assembly into the live well. One of the drawbacks of such a technique is that lubricators, which are 40 to 100 feet long, must be erected on the rig to accommodate lengthy bottomhole assemblies. This is time-consuming and expensive and further presents additional safety hazards for personnel who must be present near the top end of the lubricator to facilitate the insertion of the downhole assembly into the lubricator.
Regulations require that at least two positive shut-offs be provided from the well pressures at the surface where the downhole assembly is put together. The subsurface safety valve, which is a standard item on all the wells, is one such barrier. In some situations where the dual barrier can be required is if an existing well needs to be perforated at another location. In the past, large lubricators have been built at the rig floor to accommodate a gun assembly which could be fairly lengthy.
One of the objectives of the present invention is to eliminate the need for building lengthy lubricators at the rig floor by employing a portion of the wellbore for assembly of lengthy downhole assemblies such as perforating guns. Thus, in accomplishing the objective, the present invention provides for a second barrier such as a plug in addition to the subsurface safety valve. This additional barrier can be manipulated out of the way to allow the additional downhole function to be performed and, at the same time, the plug can be repositioned so that the assembly, which has been put together in the wellbore, can be brought up above the subsurface safety valve. Once again, two isolation devices will exist to permit the disassembly of the lengthy downhole assembly still in the wellbore. Thereafter, the upper barrier can be removed from the wellbore to facilitate future operations.
The prior art illustrates numerous styles of subsurface valves primarily used for safety shut-off purposes. Some assemblies involve singular valves and others involve dual valves. Typical of such art are U.S. Pat. Nos. Reissue 25,471; 4,116,272; 4,253,525; 4,273,186; 4,311,197; 4,368,871; 4,378,850; 4,444,268; 4,448,254; 4,476,933; 4,522,370; 4,579,174; 4,595,060; 4,603,742; 4,618,000; 4,619,325; 4,624,317; 4,655,288; 4,665,991; 4,711,305; 4,846,281; 4,903,775; 4,415,036; 4,427,071; 4,531,587; 4,825,902; 4,856,558; 4,986,358; 5,201,371; 5,203,410; 5,213,125; 5,411,096; and 5,465,786. This subject has also been written about in the November 1995 issue of World Oil in an article by Tim Walker and Mark Hopmann, entitled "Underbalanced Completion Improved Well Safety and Productivity," and in an SPE, Paper No. 304 Q1 by Tim Walker and Mark Hopmann, entitled "Downhole Swab Valve Aids In Underbalanced Completion of North Sea Well." This SPE paper was presented in the 1995 meeting held in Aberdeen.
The prior art just described reveals various components of downhole safety valve systems which include flapper-type and ball-type valves. What has been lacking is a system that is versatile and reliable as the system that is the present invention which facilitates the assembly of long downhole assemblies in the wellbore. The new system is flexible and can be readily installed when using extended assemblies in conjunction with wireline coil tubing or work string assemblies.